Many of the next generation of commercial passenger aircraft will incorporate an advanced passenger entertainment system in the cabin. In such an entertainment system, each passenger seat will be provided with an individually controllable audio receiver or visual display. The audio receiver will allow a passenger to listen to and select among several different channels of music programming. The video display will allow a passenger to play video games, select among a number of different movies or shows, or connect his or her portable computer to the video display to use as a monitor. The passenger entertainment system will therefore allow passengers to entertain themselves during long flights.
Incorporating an individualized passenger entertainment system in an aircraft is a challenging engineering problem. Multiple channels of audi must be transmitted to each of the passenger seats from a central control location. Since most commercial passenger aircraft have several hundred seats, a large network must be provided within each aircraft to allow signal distribution. The audio and video signal reception must be sufficiently strong at each passenger seat, otherwise the passenger will receive programming filled with static. Compounding the problem of designing an adequate distribution network is the variability in aircraft layout. Because most aircraft manufacturers sell many different styles of a single aircraft with a variety of seating arrangements, it is not possible to design a standard network for inclusion in all the aircraft. Seats are typically added to and removed from an aircraft during the aircrafts lifetime, changing the seating configuration within a given aircraft. As the number and location of seats change, the cable lengths in the network change and the total load on the network changes. Each change has an effect upon the signal quality of the entertainment system. A passenger entertainment system must therefore include a distribution network that is capable of dynamically compensating to account for the changing conditions that occur as the seating arrangement of the aircraft changes.
A typical passenger entertainment system resembles a small cable TV system. Each channel of audio and video programming is modulated onto an RF carrier. The RF carriers are then multiplexed onto a single transmission line at a central control station, and transmitted over a single coaxial cable bus to a series of intermediate stations. Each intermediate station is designed to provide the signal to different regions of the aircraft. At each area of the aircraft, the signal is further tapped to provide individual signals to each passenger seat. Because of the multiple splitting of the signal that occurs, the power level of the signal has a tendency to drop the further signal gets from the central control station. Additionally, the inherent resistance of the coaxial bus also dissipates the power of the entertainment signal. In order to maintain signal quality, it is therefore important to maintain adequate RF signal power levels throughout the system. In order to compensate for the loss of power due to signal transmissions, amplifiers are typically provided throughout the system components. The amplifiers amplify the entertainment signal to ensure the quality of the signal received by each passenger.
An example of a passenger entertainment system having distributed amplifiers is described in U.S. Pat. No. 5,220,419 entitled "Automatic RF Leveling In Passenger Aircraft Video Distribution System." The system includes a number of stations (18, 28) which tap and split an audio/video signal that is carried on a cable (16). Each of the stations includes a variable gain amplifier that is controlled by a microprocessor. The variable gain amplifier consists of a variable attenuator having an output connected to an amplifier of predetermined gain. The microprocessor monitors the signal level on the cable, and adjusts the attenuation provided by the variable attenuator to set the signal to the desired level. The system disclosed in U.S. Pat. No. 5,220,419 also allows the microprocessor to communicate among the various stations. If one station is unable to provide sufficient amplification to the signal due to the operating limits of the variable gain amplifier, a station located closer to the signal source increases the amplification that it provides. Several stations can therefore be networked together to provide appropriate amplification and ensure that the signal power level is sufficient throughout the system.
A passenger entertainment system constructed from a number of discrete components, such as suggested in U.S. Pat. No. 5,220,419, has several shortcomings. Because all electrical systems on an aircraft must operate from an onboard power supply, it is desirable to minimize the power consumption of any system. A passenger aircraft video distribution system having a number of discrete components will generally consume a substantial amount of power. Not only does this tax the power supply of an aircraft, but the corresponding generation of heat by the system requires further compensation by the aircraft climate control to maintain passenger safety and comfort.
Each part included in a passenger aircraft video distribution system contributes to the cost of the system. The inclusion of multiple discrete components at each station in the system has a tendency to further increase the overall system price. Because an aircraft will typically have several hundred seats, a passenger entertainment system can become prohibitively expensive if costs are not kept to a minimum.
Finally, the number of discrete components contained in a distribution system adds to the physical size of the system. As those who have flown on a commercial aircraft will attest, the amount of space in and around the passenger seat is limited. Any passenger entertainment system incorporated into the seats should therefore encompass as small a space as possible. Not only does this increase the comfort of the passenger, but any weight savings that are made translate into a greater operating efficiency for the aircraft itself.
The present invention is directed to providing an automatic leveling circuit for a distribution network of a passenger aircraft entertainment system that overcomes or minimizes the above-mentioned problems.